Power quality related problems, in particular voltage sags and brownouts, have a major negative impact on industrial productivity. This appears to be true for both industrialized as well as developing nations. There are several distinct solutions for voltage regulation including servo-stabilizers, CVT’s, ferroresonant regulators, thyristor ac regulators, tap changers and more recently, electronic regulators. This paper briefly examines commercially available technologies, and compares performance attributes of the various solutions for industrial and commercial use. Important comparison points include speed of response, types of faults handled, ruggedness, cost and efficiency.
Power Quality Issues in Industrial Plants:
As industrial plants move towards a globally competitive environment, achieving and sustaining high levels of productivity becomes a critical factor. For this to occur, it is important that processes operate essentially uninterrupted. Even in industrialized nations, one of the dominant reasons for process interruption is due to power quality problems with the incoming utility supply. In developing countries, the situation is substantially worse, as both the quality and the availability of electrical power are highly compromised. Power quality problems which compromise industrial productivity include transients, voltage sags and surges, brownouts, harmonics and power interruptions. Power interruptions have conventionally been handled through the use of a backup power source such as a generator or UPS. The UPS is a fairly expensive solution, as it relies on batteries for energy storage, and it has traditionally been applied to support electronic loads such as computers and industrial controllers. Very rarely is a UPS used to support an entire industrial process. The use of generators for backup power, particularly in the developing countries is prevalent. This is primarily because power outages are very common, and operation on generator power can keep the plant operational. It is important to note that in areas where there is an acute shortage of power, power outages are typically scheduled on a rotating basis. This allows industrial plants to coordinate their plant holidays, or to schedule operation with internal generators. This suggests that a more important problem is impaired industrial productivity due to the occurrence of random events such as transients, voltage sags and brownouts, and not due to power outages.
The primary focus of this paper is to examine problems and solutions which can reduce the susceptibility of an industrial plant to power quality related problems. Of the many problems listed above, the potentially most serious, and difficult to tackle, is the occurrence of voltage sags (typically 6-10 cycles), and brownouts. Voltage sags are inevitable on the power system, even in the most advanced utility networks. These are mostly caused by faults on the system, with line to ground faults being most frequent. In industrialized countries, it is very rare to have outages that extend significantly beyond the time required to clear the fault, and to re-close a breaker to connect the customer to a different part of the utility network. In fact, one of the more promising solutions which is being actively pursued for industrialized countries is the use of static transfer switches, where two independent power feeds are brought into the plant, and the plant supply can be rapidly switched from the faulted line to the unfaulted line so as to maintain power to the plant. It has been shown that this can handle over 95% of all voltage sag and utility fault related events. The static transfer switch is an expensive solution, unless the second feed is already present. In any case, it is important to note that this solution is not effective in cases where voltage sags and brownouts are due to acute power shortage (typical for developing countries), as all feeds will see similar under-voltage conditions.
Problems Due to Voltage Sags:
The impact of voltage sags on industrial processes has been extensively studied, and is the subject of numerous reports by utilities, industries and consultants. Short term (3-10 cycle) voltage sags are more frequent in the industrialized nations, while brownouts tend to be more prevalent in the developing countries. The biggest problem associated with voltage sags is the tripping of sensitive equipment such as motor drives. This occurs because of insufficient voltage across contactor and relay coils, and reduced dc bus voltages in adjustable speed motor drives, programmable controllers and other electronic loads. Some ASD’s are designed to provide automatic restart, or in some cases ride through using load inertial energy. In such cases, the ASD’s will ride through the short term sag, unless the motor is part of a precision position controlled servo process. Short term voltage sags have little impact on most direct line connected
motor loads.
Fig. 1: Voltage sag on typical utility line, and action of automatic voltage regulator
Operation of industrial equipment under brownout conditions is much more problematic than under voltage sags. In addition to all the problems encountered for short term sags, one has to contend with plant operation at reduced voltages, possibly for hours at a time. Severe problems of motor stalling, motor over heating, motor tripping, operation at improper load/speed points, and complete process shut down are likely scenarios. It is clear that in developing countries in particular, any solution proposed should offer protection under both short term sags as well as under brownout conditions.
Various solutions are presently used for correcting voltage sags and brownouts. These include tap changing transformers with mechanical or static switches, saturable reactor regulators, phase controlled regulators, ferroresonant transformers, and motorized variacs. Some of the advantages and disadvantages of using these solutions are given below.
Tap Changing Transformers:
Fig. 2: Electronic tap changing transformer
Saturable Reactor Regulators:
Fig. 3: Saturable reactor regulator
Motorized Variacs:
Fig. 4: Motorized variac
CVT’s / Ferroresonant Transformers:
Fig. 5: Ferroresonant transformer with a neutralizing winding
Phase Controlled Regulators:
Electronic Voltage Regulators:
A new class of automatic voltage regulators based on high frequency switching inverter technology as shown in Fig. 6. It can provide fast response (1-2 milliseconds), sinusoidal voltages, and compact design. This category of voltage regulators potentially offers the highest performance solution. However, designing appropriate overload capability can make the overall cost unacceptably high.
Fig. 6: Electronic voltage regulator
In order to realize a fast response and high performance electronic voltage regulator with the lower cost of the more conventional schemes, a hybrid configuration using active and passive components can be used. Using the series transformer concept, a well established and popular concept in many industrial applications, in combination with a power converter as shown in Fig. 7, a cost effective solution can be realized. This implementation is similar to the series active filter implementation reported in [6]. As indicated in [6], a bypass switch can be used to provide bypass protection in case of faults. This implementation lends itself to advanced features such as active filtering.
Fig. 7: Block schematic of a hybrid electronic voltage regulator
Soft Switching Line Conditioners:
In order to realize improved system performance, soft switching automatic voltage regulators (SSAVR) can be used. These line conditioners combine the fast response and high performance of active line conditioners with the lower cost of the more conventional solutions. The heart of the power line conditioner is an IGBT based soft switching inverter technology, such as the resonant dc link inverter (RDCLI) a high efficiency and high performance inverter (Fig. 8). Industrial grade automatic voltage regulators rated at up to 1 MVA can be realized using the soft switching approach.
Fig. 7: Circuit schematic of resonant dc link inverter used in a SSAVR
These line conditioners are also based on a hybrid configuration using active and passive components to realize a cost effective solution. The active components are coordinated to tolerate overloads occurring due to transient events such as faults and large motor starting. The SSAVR also blocks normal and common mode noise present on the utility line, and actively attempts to suppress voltage deviations on the load side of the conditioner. The EMI generated by these units is low, in spite of the high frequency switching (70 kHz) employed. The high efficiency and low EMI are primarily due to the soft switching nature of the inverter. These units can maintain the output voltage to within 1% of nominal value with a wide variation in input voltage. Response to input or load fluctuations is sub-cycle, and may be considered almost instantaneous for the industrial loads served. Efficiency of a typical SSAVR is fairly high approximately 97% at full load, significantly higher than competitive solutions.
In summary, voltage sags are becoming an increasing concern to industrial plants due to increased automation. Potential process disruptions with sensitive loads and additional labor and operational costs prompt the need for voltage regulators to provide a ride through capability. Constant voltage transformers (CVT’s) can be applied economically at constant loads to wide variety of voltage sag conditions. With non-linear loads and sensitive loads that require fast transient response, electronic voltage regulators may offer a cost effective solution. In addition, advanced features such as active filtering can be also achieved using these schemes.
For further reading:
[1] Mark McGranaghan, D. Mueller and M. Samotyj, "Voltage Sags in Industrial Systems", IEEE Trans. on Industry Applications, March/April 1993, pp. 397-402.
[2] H. Sarmiento and E. Estrada, "A Voltage Sag Study in an Industry with Adjustable Speed Drives", IEEE Industry Applications Magazine, Jan/Feb 1996, pp. 16-19.
[3] Power Line Problems in Industrial Environments, Short Course Notes, UW Madison, WI, Nov 1995.
[4] D. M. Divan, "The Resonant DC Link Inverter - A New Concept in Static Power Conversion", IEEE Industry Applications. Society Conference Records, 1986, pp. 648-656.
[5] D. M. Divan, "US Patent no. 4,864,483, Sept. 5, 1989".
[6] S. Bhattacharya, D.M. Divan and B. Banerjee, "Synchronous Frame Harmonic Isolator Using Active Series Filter," EPE’91 Conference Records, pp. 30-35.
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